The present invention relates to a brake valve and more specifically to a brake valve with a controllable intensification device for controlling the pressure ratio between a master pressure supplied by a brake pedal and a braking pressure developed in a brake.
The brakes of a vehicle assembly, consisting of a tractor hitched to one or more trailers, must be adapted to one another so that no substantial hitch forces are produced between the individual units of the vehicle. A standard has been established for coupling different vehicles of any manufacture which provides that a brake pressure of 100 bar present on a hydraulic coupling must produce a 30% braking of the vehicle.
Brake valves are known in which a control piston and a measurement piston are arranged one behind the other in a borehole so that respective end surfaces of each are in contact. The other end surface of the control piston is subjected to an input pressure generated by a brake pedal of the tractor while a resulting pressure which builds in the trailer brake is fed back and applied to the other end of the measurement piston. The application of the input pressure to the control piston displaces both pistons which are designed to thereby establish a channel between pressure fluid lines of the trailer brake and a source of hydraulic fluid under pressure until an equilibrium condition is attained.
Since the pressure applied to the brake of the trailer is to be proportional to the input pressure developed by the tractor, the surface areas of the pistons must be appropriately proportioned to achieve the required pressure ratio. Quite obviously, to change the pressure ratio, the diameter of the master piston must be changed, entailing an expensive undertaking. Furthermore, the absorption volume of the master piston is undesirably dependent on the set intensification ratio since it is a function of the diameter of the master piston. In order to maintain a small absorption volume, and also avoid the undesired effects resulting from the dependency of the shape of the master piston on the absorption volume, the stroke and diameter of the control piston must be kept to a minimum.
This leads to high manufacturing costs and/or to high leakage values due to the small overlap between the control edges in the housing and the circumferential surface of the control piston upon the opening and closing of the corresponding channels.
Federal Republic of Germany Application DE-OS 29 38 978 discloses a brake valve for controlling the brake pressure on a trailer wherein the brake is acted upon by a primary pressure from an accumulator. This pressure control relies upon the buildup or relief of the pressure taking place via a control piston whose one end surface is acted on, not only by a spring but also by the brake pressure fed back at the same time. In parallel to the control piston there is arranged a servopiston, one end surface of which is subjected to a master pressure from a brake pedal and which transmits this pressure in the form of a force to one lever arm of a double-armed lever which operates as a balance beam. The other lever arm applies the force, appropriately intensified, to the other side of the control piston. In this manner, for every master pressure, i.e. pressure supplied by the brake pedal, there is established for a certain master pressure a corresponding brake pressure which is supplied to the trailer and which is proportional to the master pressure in accordance with the leverage ratio. In this arrangement the balance beam is acted upon by the full master pressure. This pressure, in the event of panic braking, may equal 4 to 5 times the normal operational pressures. The balance beam must, therefore, be designed to meet these rarely occurring maximum pressures.